Getting my head around the "furnace" business- Any ideas?

[Jacob Beg]

Hi  All-

I have acquire a SF rental property and a MF-6 unit property. 

The SF has an oil furnace and window AC units. The oil tank is located is in the basement, there is Natural gas at the property and I am debating whether to get rid of the 30 year old functioning oil furnace as the flip market in this area mostly comprise of central air. Will this require changes to ductwork? I have never made the switch from oil to has before. Are there any better options?

For  MF, there is an oil boiler. LL is responsible for paying for heat and water. The boiler is quite old, but have a tech who says he can "switch" it to a gas kit and discard the large oil tanks for 7K. Someone else recommends against this approach due to difference between how energy is consumed by oil boiler and gas boiler and I don't really fully understand the difference. However, oil is pretty expensive. Changing to a new gas boiler is approx 14K. And there is no central mechanism, its window AC in all units. Any ideas?